A radio frequency (RF) signal line is provided in a wireless terminal such as a mobile phone. The RF signal line according to the related art is mounted in the form of a coaxial cable. In the case of being mounted in the form of the coaxial cable, it is general that the flexible circuit board is used in recent years because the space utilization in the wireless terminal is deteriorated.
Since an optimum impedance of a signal transmitting terminal of the flexible circuit board is about 33Ω, and an optimum impedance of a signal receiving terminal is about 75Ω, a characteristic impedance of the flexible circuit board may be generally designed in consideration of all the signal transmitting/receiving terminals.
When an external signal is introduced into the flexible circuit board, the above-described characteristic impedance deviates from 50Ω that is a reference value to adversely affect signal transmission efficiency. When the other part that is a conductor such as a main board, a sub board, a battery, or the like come into contact with the ground or is disposed close to the ground, the characteristic impedance deviates from 50Ω while a signal is introduced from the outside.
Thus, the flexible circuit board may be mounted on a position that is approximately spaced apart from the other part, or a dielectric is adjusted in thickness so as to prevent the impedance from varying.
An RF transmission line that transmits the high frequency signal is applied to various wireless terminals such as a smart phone, a tablet, a wearable device, and the like. In recent years, as development of communication technologies, a carrier aggregation (CA) technology, which uses one bundle of two or more frequency bands different from each other, is applied to improve a data transmission speed.
For this, two or more RF transmission lines that transmit the high frequency signal is used in the wireless terminal. The two or more RF transmission lines and a normal transmission line for transmitting normal signals such as a plurality of digital signals and analog signals, power, and the like are connected to the main board and the sub board.
As described above, when the coaxial connector is used as each of the RF transmission line to prevent signals from interfering with each other, a space occupied by the thickness of the coaxial connector itself and a fixing body for fixing he coaxial connector increases in the wireless terminal. To solve such the spatial problem, there is a need to use the flexible circuit board.
However, when the flexible circuit board is used, since the RF transmission lines different from each other have to be spaced apart from each other so as to solve the signal interference problem, there is also a limitation in solving the spatial problem within the wireless terminal.
In addition, there is a problem that the number of connector connecting the RF transmission lines to each other increases, and the length of each of the RF transmission lines becomes longer, and thus, a signal loss also increases.
It should be understood that the foregoing description of the background art is merely for the purpose of promoting a better understanding of the background of the present invention, and is not to be construed as admission that the prior art is known to those skilled in the art.